Insect Symbiosis - SGM series

It's a pretty well known fact now that the human body contains lots of bacteria. Bacteria live on your skin and in your throat and gut, for the most part completely harmlessly, protecting your body from more dangerous invaders.

But something that doesn't get mentioned quite so often is that humans are not the only animals with a corresponding posse of bacteria. Other animals have them as well, including insects. From a bacterial point of view both your body and an insect's body are merely new lands to be colonised, and if they can colonise those lands without totally destroying them, then so much the better.

Like human bacteria, some bacteria that live within insencts have formed a symbiotic relationship, where the insect relies on the bacteria for survival. The pea aphid (shown to the right - photo by Marlin E. Rice) contains a symbiotic bacteria, Buchnera aphidicola that is required to produce one of the major amino-acids used to make important proteins. Completely sequencing the genome of the aphid shows that it does not contain the gene for argenine; it requires the Buchnera to make it. Likewise the sequenced bacterial genome lacks the genes for animo-acid deregulation, and several other minor amino acids. It gets these from its insect host. The bacteria lives within the host, in specialised little cells, and is passed down from mother aphid to daughter as without it the aphids will not survive.

This raises important questions about the control of the genetic activity of both the bacteria and the insect. If the insect needs more argenine, it must have a way of telling the bacterial genome to produce it, likewise if the bacteria requires more of the non-essential amino-acids it needs to be able to push the insect to make them. Modeling flux pathways for the creation and degredation of some of these amino acids helps to build up a picture of how this control can function, at a metabolic level if not a genomic one. The flux analysis also shows how important this symbiotic relationship is, for both the bacteria and the insects.

Leaf cutter ants (shown on the left - image from Wikimedia commons) have what is possibly the most complex and fascinating of interactions with microorganisms. For a start, they harvest fungi growing it in little gardens and feeding it with mashed up plants. This fungi can be susceptible to infections, so the ants also need to provide pesticides to keep their crops alive. As ants have not quite reached the level of large scale chemical manufacture, they have to rely on symbiotic bacteria to produce the antibacterial and antifungal compounds they need. The bacteria they use are species of Pseudonocardia and Streptomyces which produce a large number of secondary metabolites that can be used to destroy the fungal-infectors. The ants excrete these secondary metabolites in their waste, which can then be moved into the fungal garden. The bacteria also showed some anti-fungal activity against the fungus growing in the gardens, so could be used to control how far the crop spreads.

I'm always wary of ants, I certainly got bitten by them enough times as a kid. With their little societies and gardens and wars and multistory-housing compexes they are scarily human for a tiny piece of exoskeleton with legs.

Schoenian I, Spiteller M, Ghaste M, Wirth R, Herz H, & Spiteller D (2011). Chemical basis of the synergism and antagonism in microbial communities in the nests of leaf-cutting ants. Proceedings of the National Academy of Sciences of the United States of America, 108 (5), 1955-60 PMID: 21245311

I think with the aphids it's probably only a matter of time before the internal bacteria become more like a specialised organelle. I don't think anyones done any studys yet on how able the bacteria or aphids are able to survive alone, but it can't be too healthy!